- A 2-centimetre heated steel sphere cooling in boiling water. In the left image the sphere is in the film-boiling or Leidenfrost regime, wrapped in a vapour layer. In the right image the sphere temperature has fallen and the cooling is switched to nucleate boiling regime (Image: Ivan Vakarelski)
- mg21528825.000-1_300[1].jpg (12.22 KiB) Viewed 2324 times
The phenomenon is based on the Leidenfrost effect. If a frying pan is hot enough, droplets of water will skitter around rather than spreading into a puddle. That is because high heat evaporates enough of the water to create a vapour layer. The liquid floats on this cushion and boils without bubbling, says Neelesh Patankar of Northwestern University in Evanston, Illinois. As the surface cools, the vapour layer can collapse and the water will bubble, triggering a violent outburst.
The Leidenfrost effect is a concern in chemical plants and nuclear reactors, where liquid water touching hot metal may cause explosions.
But if hot water can be kept away from the material long enough, the team hypothesised, the vapour might stay in place past when the material cools to the boiling point of water, and there is no more explosion risk.
To test this, the team covered steel balls in a nanoparticle-based coating that gave them a rough texture (Nature, DOI: 10.1038/nature11418). They heated the balls to 400 °C and submerged them in hot water.
Rather than bubbling against the scorching metal, water droplets stretched across the grooves in the rough coating and cavities beneath them filled with vapour. This kept the surrounding water undisturbed as the temperature of the balls fell all the way to 100 °C.